A Navigation satellite system (NSS) is a space based radio positioning network for providing users equipped with suitable receivers highly accurate position, velocity, and time (PVT) information. One of the most recognizable NSS systems is the global positioning system (GPS) developed by the United States of America, although there are numerous other systems including local NSS's that utilize fewer satellites in a geosynchronous orbit. Thus, although the following background describes the operation of the GPS system in particular, it is appreciated that the description is meant to provide a generic overview of NSS operations as opposed to specific reliance on a single NSS.
In general, the space based portion of GPS comprises a constellation of GPS satellites in non-geosynchronous 12-hour orbits around the earth. GPS satellites are located in six orbital planes with four of the GPS satellites in each plane, plus a number of “on orbit” spare satellites for redundancy. The orbital planes of the GPS satellites have an inclination of 55 degrees relative to the equator and an altitude of approximately 20,200 km (10,900 miles) and typically complete an orbit in approximately 12 hours.
The positions of GPS satellites are such that a minimum of five of the total constellation of GPS satellites are normally observable (above the horizon) by a user anywhere on earth at any given time.
NSS position determination is based upon a concept referred to as time of flight (TOF) ranging. For example, each of the orbiting GPS satellites broadcasts spread spectrum microwave signals encoded with positioning data and satellite ephemeris information. Essentially, the signals are broadcast at precisely known times and at precisely known intervals. The signals are encoded with their precise time of transmission.
The signal is received at a NSS receiver designed to determine the time of flight from satellite to receiver by synchronizing a local replica of the satellite signal and determining the time of flight of this signal, compare it to the time of transmission, and to demodulate the satellite orbital data contained in the Satellite Data Message. The coded satellite signal contains information describing when the signal was sent from the satellite. The NSS receiver determines the time between transmission of the signal by the satellite and reception by the receiver. Multiplying this by the speed of light gives what is termed the pseudorange measurement of that satellite. It is called a pseudorange because of the many kinds of errors that occur due to clock errors, propagation errors, multipath errors, and the like.
If the NSS receiver clock were perfect, this would be the range measurement for that satellite, but the imperfection of the clock causes it to differ by the time offset between actual time and receiver time. Thus, the measurement is called a pseudorange, rather than a range. However, the time offset is common to the pseudorange measurements of all the satellites. By determining the pseudoranges of four or more satellites, the NSS receiver is able to determine its location in three dimensions, as well the time offset. In addition, because the signal is consistently generated and broadcast, the NSS receiver is able to determine position, velocity, and time (PVT) information on an ongoing basis thereby allowing navigation from point to point.